1. Field of the Invention
This invention relates to a bonding tool used, in particular, as a compression bonding tool for wireless bonding of a TAB system (Tape Automated Bonding), etc. in mounting semiconductor elements of IC and LSI, etc. on substrates, etc., or in bonding semiconductor elements of IC, etc. to electrode wires, a process for the production of the same and a method of measuring the flatness of a bonding tool at an application temperature.
2. Description of the Prior Art
Lately, technical progress in the field of semiconductors has become remarkable and production of the appliances using IC or LSI has shown a yearly increase. In order to draw out the electrical properties of these semiconductors, it is required to bond an electrode formed on an IC with a metal-plated lead or a metallic fine wire called a bonding wire. As the metal to be bonded, there is ordinarily used Au or an Au-Sn alloy which is chemically stable and has high electric conductivity, and a bonding method comprising thermocompression a bonding by means of bonding tool heated has widely been used.
The large-size and high density tendency of IC chips with the increase of functionality of IC results in a requirement that a bonding tool should be large-sized and the tool end surface should be so flat as to be represented by a degree of flatness of at most 1 .mu.m at an application temperature in order to uniformly press electrodes and leads formed around an IC with high precision.
In the bonding tools produced by the process of the prior art, if the end surface dimension exceeds 10 mm square, an excellent degree of flatness of at most 1 .mu.m can be obtained at normal temperature, but when the bonding tool is heated to a real application temperature selected from a range of 200.degree. to 650.degree. C., a warp occurs at the end surface of the tool due to a difference in coefficient of thermal expansion between a material such as diamond used for the end part of the tool and a metallic material used for a shank so that no flat surface is obtained. In mounting the IC, therefore, compression by the bonding tool is partly ununiform to cause a poor connection referred to as "open".
In order to mount a semiconductor element in a form of capable of practically use and to draw the electrical properties thereof, it is required to electrically connect an electrode formed on the semiconductor element with a lead of a package and electrically connect this lead with an external terminal of a printed wiring plate.
Connection of an electrode of a semiconductor element with a lead of package has been carried out by a wire-bonding method comprising connecting lead wires (bonding wires) of metallic fine wires consisting of gold or copper, one by one, by the use of a tool called a capillary. Of late, furthermore, wireless bonding of a TAB system, etc. has been widely used and comprising using a lead called a film carrier tape composed of a pattern-formed copper foil plated with gold or tin, and collectively connecting patterned leads on the film and electrodes of a semoconductor element by the use of a bonding tool heated at a predetermined temperature.
The bonding tool of the prior art, used in wireless bonding, etc. of a TAB system, etc. consists of a metallic single substance such as Mo, Fe--Ni alloys, Ni alloys, Ti or Ti alloys, W or W--Ni alloys, W--Cu alloys, Fe--Ni--Co alloys, cemented carbides and the like and has generally been used while heating at a predetermined temperature by directly passing electric current or inserting a heater. However, the bonding tool consisting of such a metallic single substance has the disadvantages that the flatness of the end pressing surface is not good, the temperature distribution is not uniform and the wear resistance is too inferior to maintain the service life of the tool.
In order to obviate the disadvantages of the bonding tool consisting of the metallic single substance, there have lately been used, in many cases, bonding tools each having the end pressing surface composed of a hard material such as polycrystalline diamond, single crystal diamond, sintered polycrystalline diamond, sintered polycrystalline cubic boron nitride (cBN), etc. For example, polycrystalline diamond is formed on a base plate consisting of a sintered compact of Si.sub.3 N.sub.4, SiC or AlN, as a predominant component, or Si by a low pressure gaseous phase synthesis method and bonded with a shank of the bonding tool. Another hard material is often fitted to the shank as it is.
Lately semiconductor chips have tended to be large-sized or long due to multifunctional high integration and realization of a semiconductor chip with 20 mm sqaure or more is supposed. With the this recent tendency of semiconductor chips the end pressing surface of the above described hard material is necessarily enlarged in the bonding tool. Further with this tendency of the semiconductor and bonding tool as described above, it has increasingly been difficult to bond all electrodes on a semiconductor chip and all leads on a film tape carrier with high reliance.
That is, for bonding all electrodes on a semiconductor chip and all leads on a film tape carrier with high reliance, it is necessary to uniformly press all the leads on all the electrodes by means of a bonding tool and to this end, a relative gap allowable between the leads superimposed on the electrodes and the end part of the bonding tool should be at most 3 .mu.m, ideally at most 1 .mu.m. When the end pressing surface is completely flat, for example, dispersion of (or differences in) the heights of the electrodes and leads, in summation, is generally allowed by upto 3 .mu.m and, conversely when there is no dispersion in the sum of the heights of the electrodes and leads, the degree of flatness of the end pressing surface can be up to 3 .mu.m. If the dispersion or the degree of flatness exceeds 3 .mu.m, uniform pressing of the end pressing surface is impossible and results in locallized peeling or lowering of the bonding strength between the electrodes and leads.
Thus, based on the recognition that there is little dispersion in the heights of the electrodes on the semiconductor chip and the leads on the film in the production thereof, only improvement of the degree of flatness of the end pressing surface in the bonding tool is expected and continued under the size-increasing tendency of the end pressing surface. Specifically, it has been considered that in order to uniformly press all of an electrode 2 and lead 3 on a semiconductor chip 1, provided in such a manner that there is no dispersion in heights as shown in FIG. 4 and they are stacked to be substantially flat, the degree of flatness of an end pressing surface 6 in a tool end part 5 provided at the end of a bonding tool 4 should be at most about 1 .mu.m. That is, the end pressing surface 6 in the tool end part 5 formed of a hard material mainly consisting of diamond or cBN should have a central part formed in a somewhat outward or curved convex manner and this end pressing surface 6 in the convex form should have a degree of flatness in the range of at most about 1 .mu.m (i.e. a 1 .mu.m difference in height between the central part and peripheral part). Various efforts have been made to this end.
In order to obtain a bonding tool having such a good flatness, there has been employed a method comprising polishing the end pressing surface of a tool end part at normal temperature, for example, by lapping, etc. up to the present time, and it has been possible to finish the end pressing surface with a degree of flatness of at most 1 .mu.m, measured at normal temperature. The measurement of the degree of flatness of the end pressing surface in the bonding tool is carried out by the use of a contact type flatness measurement apparatus in which a mechanical displacement of a contact needle is converted into an electric signal and detected, a non-contact type flatness measurement apparatus of in which a displacement of a distance is detected by measurement of a reflected wave using a laser, e.g. He--Ne laser or semi-conductor laser, or a laser interferometer.
Even if the tool end part of a bonding tool is worked so that the dgree of flatness of the end pressing surface at most 1 .mu.m measured at normal temperature by a laser interferometer, however, there have occurred a number of problems due to the large size of the semiconductor chips including, for example, deterioration of bonds between electrodes and leads or other tendencies of electrodes and leads to separate from one another once bonded, when semiconductor chips are really mounted.
As one technique of mounting an IC, there is a system of continuously bonding electrodes formed on the IC with lead wires formed on a film carrier tape using a bonding tool, which is called the "TAB system". In a bonding tool for TAB used in this TAB system, as a tool end part to be provided at the end for pressing, there is ordinarily used gaseous phase-synthesized diamond, single crystal diamond, diamond sintered compact, binderless polycrystalline cBN and polycrystalline diamond film on a substrate of Si, Si.sub.3 N.sub.4, SiC, AlN, etc., and in order to uniformly bond a number of electrodes formed on the IC and lead wires formed on a film with a sufficient strength, it is required that the surface of the bonding tool for the TAB (i.e. surface of the tool end part) is smooth and flat without undulations and inclinations.
Since the apparatus for measurement of the degree of flatness is based on a base plate with a degree of straightness and flatness and finished with high precision, and an instrument sensitive to temperature, such as optical primary standard, it is required to carry out the measurement in an isothermal chamber always maintained at a constant temperature and to maintain an object to be measured, i.e. bonding tool for TAB at or near room temperature.
On the other hand, mounting of an IC by a bonding tool for TAB is generally carried out while heating the bonding tool for TAB at about 200.degree. to 650.degree. C. Accordingly, it is desired to know the degree of flatness of the bonding tool for TAB at this practical temperature, but the standards of the base plate or optical primary standard get out of order at such a high temperature and precise measurement cannot be carried out.